1. Field of the Invention
This invention relates to a method of using heat to propagate fissure depth of a score.
2. Discussion of the Prior Art and Technical Problems
A glass ribbon is displaced downstream from an annealing lehr through scoring and severing stations to (1) remove the bulb edge, i.e., the edge of the glass ribbon engaged during formation thereof to control the ribbon thickness, and (2) cut the glass ribbon into caps or lehr ends, i.e., sections of the ribbon having the bulb edge removed and cut to a predetermined length as measured from the leading edge of the ribbon. There are many techniques for scoring and severing glass taught in the prior art. For example, U.S. Pat. No. 3,730,408 teaches the use of hot air to deepen the fissure depth of a score to about 80% of the glass thickness to minimize the bending moment forces required to sever the glass. Although this technique is acceptable for severing glass sheets, there are limitations when it is used to remove bulb edges from a glass ribbon.
The limitation of using hot air to deepen the fissure depth of the score imposed on the bulb edge is that only 20% of the glass thickness holds the bulb edge to the ribbon. Therefore, the severing of the bulb edge can result from mechanical vibrations which generate bending moment forces about the score. The mechanical vibrations may be caused by a snap roll used at a lateral score opening station. The snap roll is a roll that raises the glass ribbon above the plane of the conveyor to apply bending moment forces about the lateral score line to sever the glass ribbon into lehr ends having bulb edges. The bending moment forces to open lateral scores, i.e., scores across the width of the ribbon, are generated by the snap roll applying an upward force and the weight of the glass about the snap roll applying downward forces. After the lateral score is opened, the snap roll drops to the plane of the conveyor. The glass ribbon continues to advance along the conveyor until the next score line is in position for opening.
It has been found that raising the glass ribbon above the plane of the conveyor and thereafter dropping it to the plane of the conveyor sets up mechanical vibrations which are transmitted along the score lines imposed in the bulb edge of the ribbon. These vibrations have been found to act as bending moments which cause the bulb edge to prematurely separate from the ribbon. In addition to the obvious safety problem, the other problem is that the remaining pieces of bulb edge are difficult to remove. This is because the bulb edge as it snaps off the ribbon due to mechanical vibrations veers away from the score line(s) out to the ribbon edge. When this occurs, the pieces or fragments of glass remaining at the bulb edge are difficult to remove using automatic score opening equipment. This is because there is insufficient bulb edge width and length remaining that can be engaged to apply bending moment forces. Although the technique of removing bulb edges using hot air heaters to deepen the score is acceptable, the limitation of premature snapping of the bulb edge should be overcome for the above discussed reasons.